June 17-18, 2014 - Severe Weather
A cold front moving across NY, combined with afternoon heating, spawned a QLCS that tracked quickly across much of the state producing a wide swath of wind damage from Lake Ontario to the MA border, even including a tornado touchdown in Oneida County.
Synoptically, the aforementioned cold front was part of a northwest flow regime, with a strong piece of shortwave energy moving through the Great Lakes. The forecast area was in the right entrance region of a modest anticyclonically-curved 300mb jet streak, along with a broad diffluent height field that promoted lift. The 12z KALB sounding was not very unstable, not suggesting a high probability for severe weather, but did favor a showers/thunderstorms scenario as enough daytime heating could yield ~1000 J/kg of SBCAPE and 0-6km shear values of 40-45 knots easily could support at least scattered convection. The wind field was largely unidirectional with modest midlevel lapse rates ~6 C/km. DCAPE values were respectable around 825 J/kg. Upstream at KBUF, their sounding was slightly more favorable thermodynamically, with DCAPE values approaching 1000 J/kg and midlevel lapse rates approaching 6.5 C/km. 0-6km shear values were lower around 30 knots. Both soundings were quite warm, with FZL heights ~13kft and -20C heights ~24kft. With modest boundary layer heating expected during the day and modest shear, the severe threat (if any at the time), favored more of a wind threat if a cold pool could become strong enough. While hail was still possible, it wasn't a primary threat given the warm soundings.
A few cells developed across western NY during the early to mid afternoon hours and based on how far the convection was from the synoptic forcing, it was determined that a severe thunderstorm watch was not needed for our forecast area. Additional convection initially fired up ahead of the cold front in southern Ontario (closer to the shortwave) during the late afternoon hours. This convection congealed into a cluster of multicells that weakened little despite traveling over the cool water of Lake Ontario. Once this convection got east of the lake, it interacted with a remnant lake breeze boundary which helped enhance and focus the convection. By this time in the evening hours as well, shear values had increased to 40-50 knots as the shortwave moved closer to the region.
SPC then issued a new Severe Thunderstorm Watch to account for these cells containing a wind threat as the mode quickly transitioned to QLCS. Several reports of wind damage began to come in from BUF and BGM, with BUF even issuing a Tornado Warning near the KTYX radar site for a loosely-organized bookened vortex that spinned up along the line. Velocity values from this line, especially near KTYX, were strong with inbound values as high as -60 to -65 knots less than 1500ft AGL. Most of the reflectivities in the storms were shallow, with 50 dBZ echoes barely reaching the -20C level except for a rogue cell that formed ahead of the line and had 50 dBZ echoes as high as 35 kft (produced 1 inch hail).
A Severe Thunderstorm Watch was then issued for parts of the forecast area from the Capital District and points north and west, with the eastern extent being the VT border. The northern edge of the QLCS reached northern Herkimer County first, with the first severe thunderstorms observed there. We actually were able to get verification by calling some places in Old Forge. As the QLCS reached the Mohawk valley, it began to accelerate as terrain-channeling effects took hold. Southern Herkimer County was hit primarily hard, with widespread reports of wind damage. Several other locations in the Mohawk valley received wind damage as well. Reports stretched into the Capital Region and across portions of Columbia, Berkshire, and Bennington counties as well. The density of reports dramatically decreased from the Hudson River Valley eastward with the diminishing instability in place.
Throughout the duration of the event, the QLCS exhibited a high frequency of lightning strikes, which may have also caused tree and power line damage as well. Since the line was moving, no flooding occurred despite some locations quickly receiving up to an inch of rain in less than an hour. The wind damage resulted in power outages as well, particularly across the Mohawk Valley.
Although the thermodynamic setup wasn't ideal for a derecho-type event, a sufficient QLCS can still be maintained by its own cold pool, which may have been the driving force behind the squall line. It appeared to be two QLCS segments that evolved into the classic squall line, one of the better squall lines in the Northeast in quite some time.
Nocturnal convection doesn't always produce widespread severe weather, but when a well-forced QLCS is in place, it is possible, despite whatever time of day it may be and despite the somewhat marginal convective parameters in place. BJF
The maintenance of the squall line was impressive with instability values (surface based of 500-1000 J/kg) across much of the region prior to midnight. One of the biggest challenges is when to stop warning on the line with the waning instability and developing nocturnal low-level inversion. A few reports were missed in the Hudson River Valley and over the southern Green Mountains. The line tended to redevelop over Columbia County into the southern Berkshires perhaps due to more instability in place and low-level flow enhancement or convergence off the southern Taconics.
Above: Storm Prediction Center day 2 severe weather outlook and probability for damaging wind. Note the northeastern U.S. was not outlooked.
Above: Loop of Storm Prediction Center day 1 severe weather outlooks. The northeastern U.S. was upgraded to a slight risk.
Above: Loops of probability for damaging wind, hail and tornadoes for day 1 from the Storm Prediction Center. The greatest chances were for damaging winds, although there was a non-zero probability for hail and perhaps a tornado.
Above: Upper air analyses from the Storm Prediction Center at 850 hPa, 500 hPa and 250 hPa. Upper dynamics were not remarkable but an upper level impulse and low-level jet energy were approaching, seen in the 850 hPa plot.
Above: Upper air soundings from 1200 UTC 17 June from Albany, NY (KALB), Buffalo, NY (KBUF), and Upton, NY (KOKX). There was definitive instability at all three sites and fairly unidirectional west to west-northwest flow.
Above: Upper air soundings from 0000 UTC 18 June from Albany, NY (KALB), Buffalo, NY (KBUF), Chatham, MA (KCHH) and Upton, NY (KOKX). The atmosphere remained unstable with increasing deep moisture at kalb and kbuf into the evening.
During the event
Above: Severe Thunderstorm Watches issued over the region.
Above: Loops of Storm Prediction Center mesoscale analyses of surface based CAPE, deep shear, low level moisture convergence and midlevel lapse rates. The centers of maximum shear, maximum instability and moisture convergence were over much of NY during the late afternoon then build south and east through the evening. The midlevel lapse rates were 6.5C/Km to 7.5C/Km, which in past studies has been shown to support enhanced updrafts in deep convection and increased coverage of observed severe weather.
Above: Water vapor satellite imagery loop.
Above: KENX radar reflectivity loop and KENX radar base velocity loop. Notice the line of thunderstorms in the reflectivity and the strong winds associated with the storms in the base velocity, especially within the bowing segments.